This invention relates to an elevator control apparatus which employs an induction motor, and more particularly to a control apparatus for elevators which eliminates a magnetic flux ripple ascribable to the air gap fluctuation of an induction motor caused by the deflection of a shaft.
In recent years, an induction motor has been utilized as the electric motor of a hoisting machine for elevators, and an inverter of the variable voltage and variable frequency system has been used as means for controlling the speed of the induction motor over a wide range from the stop state to the full speed thereof.
FIG. 1 is a half sectional view showing a conventional hoisting machine for elevators which employs the aforementioned induction motor as a driving source. Numeral 1 designates the bed of the hoisting machine, and numeral 2 the induction motor installed on the bed 1. This induction motor 2 includes a stator 2a and a rotor 2b, which is fastened to a shaft 3 arranged on the center axis of the induction motor 2. One end of the shaft 3 is mounted on a bracket 2c unitary with the induction motor 2 so as to be rotatable through a bearing 4, while the other end of the shaft 3 is extended out of the housing of the induction motor 2 and is mounted on a bracket 5 installed on the bed 1 so as to be rotatable through a bearing 6. In addition, a sheave 7 is mounted round that part of the shaft 3 which lies between the induction motor 2 proper and the bracket 5. Further, a brake drum 8 is integrally formed at one side edge of the sheave 7, an arm 9 having a brake shoe 9a is disposed on the outer peripheral side of the brake drum 8 so as to be freely opened or closed, and the brake arm 9 is actuated by a brake magnet 10.
In the hoisting machine constructed as stated above, the cage and counterweight of the elevator are suspended by a main rope wound round the sheave 7, so that the load of the elevator acts on the sheave 7. In consequence, the shaft 3 supported at both the ends deflects downwards. When the shaft 3 deflects downwards, the air gap Ga between the stator 2a and rotor 2b of the induction motor 2 becomes larger on the upper side and smaller on the lower side as shown in FIG. 2.
Accordingly, a rotating magnetic flux to be generated by affording a magnetomotive force of identical magnitude to the induction motor is maximized when it passes through a path of low reluctance, that is, when the sense thereof is downward as indicated by an arrow or upward as viewed in FIG. 3. When the magnetic flux is in the leftward or rightward sense, it is minimized because it passes through the air gap part of high reluctance over a longer distance. This situation is illustrated in FIG. 4.
FIG. 4 represents the angle of a magnetic flux vector (zero for the downward flux vector in FIG. 3) on the axis of abscissas, and the magnitude of a magnetic flux on the axis of ordinates. Relative to a reference magnetic flux indicated by a straight line I, the magnetic flux vector attendant upon the deflection of the shaft is in the shape of a sine wave which fluctuates by 2 cycles during one revolution of the rotor 2b as indicated by a curve II.
A torque T which the induction motor generates is given by: EQU T=i.sub.2g .multidot..PHI.
where i.sub.2g : the component of a secondary current orthogonal to a rotating magnetic field. Therefore, when the magnetic flux .PHI. has the ripple as the curve II in FIG. 4, the torque oscillates at the same ripple frequency to drastically worsen a comfortable ride in the elevator.
For the purpose of obviating the above problem, it is considered to increase the strength of the shaft 3 so that the shaft may not deflect even when subjected to the elevator load. This, however, necessitates designing the shaft to be thick and cannot be said practical on account of increase in the weight of the hoisting machine, etc.